Dear @ apoorvm, My apologies for delay in replying. Being on the move, I could not attend your quarry.
Seems your mother has a peculiar blessings from the ancestors. But your wish is like you have blooming lawn and standing by the window, you want only the fragrance of the flowers to enter your room. But all the same, you don’t want the smoke from beyond the fence to enter inside. Tell me is it possible?
Energy is Energy…..formless and free. It is same for Positive as well as Negative. Since your mother is sensitive and acceptable to free flowing energies, she might be serving as medium of interaction by your ancestors; but like I said, the window is open and free flowing energies – whether the pure ones or the unwanted ones, both may come in.
Now, to protect, Nana ji, who again is a free energy has to come back to protect. In my opinion, in-spite freed from the world, this energy keeps on coming back, because it still takes the responsibility to guard his daughter. Please re-think, should we hold a positive energy toward Moha and put it to task again and again? Instead, your mother should rise in spiritual levels to be able to communicate to energies rather than being a medium to it.
Start healing as I have suggested above. May be it closes the window for all. But her spiritual upliftment may enable her to interact at her own will. That will close the doors for unwanted energies for ever.
Do share your experience.